Background: The THD Flexi-50 provides the ability to set the power tube bias to just about anything. There is a factory specification printed on the back of the amp that will work for all of the tubes that are possible in this amp. But is that the optimum bias? In reality, each tube type likes a specific bias, but manufacturing an amp that accepts a long list of tubes requires that a compromise setting be specified so that no one burns up their power tubes. So what IS the optimum bias for a given tube? My Flexi-50 still has its original EL-34 power tubes, so these tests represent the tones available from that setup. It is generally acknowledged that EL-34's can tolerate up to about 40mA of bias, but at that level you can expect degraded tube life.
These are all 160Kbps stereo mp3 audio files. They are each 1.0 to 1.5 Megabytes.
| 25mA Bias Level | 30mA Bias Level | 35.6mA Bias Level |
| 35.6mA_NeckPup_at6.mp3 | ||
| 35.6mA_NeckPup_at10.mp3 | ||
| 35.6mA_BridgePup_at6.mp3 | ||
| 35.6mA_BridgePup_at10.mp3 |
These were recorded with the THD Flexi-50 Amp as follows:
Don Grosh Bent Top Custom > THD Flexi-50 > THD Hot Plate > THD 2x12 Cab > Shure SM-57 and Shure Beta 52. The mics were brought in to an Alesis Studio 32 mixer plugged in to an Echo Layla interface to my PC. The recording software was Vegas Audio, and the post production was done with Sound Forge. The SM-57 was in front of the Longhorn, off to the side, and the Beta 52 was up to the bass port on the back, dead center. Post production consisted of normalizing each track so that the loudest peak on the recording was at 0db. This resulted in the louder recordings seeming lower in volume due to the larger transients on the bass port. Use your volume control to compensate. I also used Sound Forge to do the conversion to .mp3 format. The SM-57 will be on one side, the Beta 52 on the other so that you can listen with and without the bass port in the mix. No compression or equalization was employed.
The Flexi used the following settings:
Input: High
Volume: 1:30 o'clock
Treble: Noon
Bright: Off
Middle: Dimed
Bass: 2 o'clock
Boost: On
Gain: 3 o'clock
Tone: Dimed
Master: On
Master Level: 3 o'clock
Cut: 9 o'clock
Power: 50 Watt
The Flexi is equipped with the original factory tubes in this order: 12AX7 > 12AT7 > 12AX7 > EL34's (pair).
The Hot Plate was set as follows:
All switches on. Level set to -4db.
The guitar was set with all controls at max except volume. For all three bias levels, the guitar was recorded with volume at 6 and at 10 (see file names).
Conclusions? I need to woodshed some more! The Flexi sounds great at any bias settings! But I'm sure I'll pick one from these, as I listen to them and get picky.
2006-06-19 Clean Flexi Test
Someone asked if the Flexi can sound big fat and clean like a Fender. Here's my attempt:
The Flexi used the following settings:
Input: Lo
Volume: 2:30 o'clock
Treble: Dimed
Bright: On
Middle: Noon
Bass: Dimed
Boost: Off
Gain: ---
Tone: ---
Master: Off
Master Level: ---
Cut: Noon
Power: 50 Watt
Tubes:
V1: 5751
V2: 12AT7
V3: 12AX7
Power: 6550 (Tung Sol Reissue)
2006-06-20 Flexi/Twin Compare
One more pass at comparing the Flexi's clean capabilities to the Cyber Twin set for 1965 Twin Reverb.
PRS Singlecut > Radial JX-2 > Flexi/Cyber Twin (CT set for 1965 Twin Reverb)
The Flexi used the following settings:
Input: Lo
Volume: 2:00 o'clock
Treble: Dimed
Bright: On
Middle: 2:00
Bass: 3:30
Boost: Off
Gain: ---
Tone: ---
Master: Off
Master Level: ---
Cut: Noon
Power: 50 Watt
(Reverb comes from Sound Forge 5)
Tubes:
V1: 5751
V2: 12AT7
V3: 12AX7
Power: 6550 (Tung Sol Reissue)
The Cyber Twin used the following settings:
Patch: 1965 Twin Reverb
Trim: 5
Gain: 4
Volume: 4 1/2
Treble: 7
Middle: 4
Bass: 6
Presence: 9
Reverb: 3
Master: 10
4/30/07
Flexi into Alnico Golds
Setup: PRS Singlecut > THD Flexi-50 > THD Hot Plate > THD 2x12 Cab loaded with Celestion Alnico Golds (4 ohms)
Input: Hi
Vol: 2 o'clock
Bright: Off
Treb: 2 o'clock
Mid: Dimed
Bass: 4 o'clock
Boost: On
Gain: Dimed
Tone: Dimed
Master: On
MV: 2:30 o'clock
Cut: Noon
3/4 of clilp with transformer set to 4/8/16 - at pause transformer switched to 2/4 setting.
The clip:
6/30/07 Flexi with Crunch Box
Grosh Tele > MI Audio Crunch Box > Flexi-50 > Airbrake > Weber Ceramic Blue Dog
Crunch Box Settings:
Volume: Noon
Tone: 9 o'clock
Gain: Noon
Flexi Settings:
Input: Hi
Vol: 2 o'clock
Bright: Off
Treb: 2 o'clock
Mid: Dimed
Bass: 2 o'clock
Boost: On
Gain: 3 o'clock
Tone: Dimed
Master: On
MV: 3 o'clock
Cut: 10 o'clock
The clip: